1. Field of the Invention
The present invention relates generally to a three-motor type tape recorder and more particularly, to a braking device for such a tape recorder, wherein the braking is effected by an electromagnetic action, and a directional braking torque ratio, that is, a difference in braking torques for opposite directions of motor rotation, is obtained by connecting various combinations of rectifiers and resistors in parallel with the motors.
2. Description of the Prior Art
The brake of a tape recorder has to provide such features as not damaging or breaking the tape while at the same time avoiding slackening of the tape, and also stopping the running of the tape in as short a time period as possible.
One type of prior art brake is a mechanical, frictional brake. The braking force produced by a frictional brake is essentially independent of rotational direction of the reel shaft, and a substantially constant braking power is obtained for both directions of rotation. A directional difference in frictional braking is obtained in the cases where two kinds of brake shoes are used or a block braking is adopted. In such a case, tape slackening is prevented and a quick stopping can be effected, but the effect of environmental conditions on the coefficient of friction between the brake lining and the brake drum, the variation thereof with the lapse of time, and the heat produced by the friction loss are all great, all of which factors create a problem of instability whereby the tape is liable to be damaged.
Electromagnetic braking is another known type of braking. There are two types of electromagnetic braking. In one type, the driving torque is braked; in the other type a reverse torque is applied to the rotating reel shaft. In the first type where the torque is braked, an electric power generating brake is utilized, and the braking torque is proportional to the number of revolutions of the reel shaft, but because of the absence of directional difference in the braking, there is a tendency of producing slackening in the tape. However, regarding the braking itself, it is extremely stable as it is not affected by the environmental conditions, variation with lapse of time, or heat generation.
On the other hand, in the second type wherein a reverse torque is applied to the rotating reel shaft, the reverse torque is produced by reversing the polarity of the supply power source, so that, unless the power source is interrupted at the moment when the reel is stopped by the reverse torque, the reel begins to rotate in the reverse direction. Thus, the timing of the interruption of the reversed polarity power source must be extremely accurate, thereby requiring a complex mechanism. However, a directional difference in braking force is obtained in this type of system, and it also has the same advantages as the torque braking system of the first type.
As described above, frictional braking and electromagnetic braking each have several advantages and disadvantages, so that, even when both of the systems are used together to compensate for these disadvantages, increased cost cannot be avoided.